This invention relates to a method for determining the velocity vector, speed and direction, of the motion of a moving object by directing two coherent light beams onto the moving object and analyzing the two directional components in the time-space cross-correlation of the moving speckle patterns produced by the light directed onto the object, and to an apparatus to be used for the working of this method.
A number of methods have heretofore been developed for the purpose of determining the scalar velocity of a moving object without mechanically contact the moving object. One of these, called the "Doppler method for velocity measurement", makes use of the Doppler effect produced by use of a laser beam. Specifically, this Doppler method determines the velocity of a moving object by directing a laser beam onto the moving object, causing the resultantly scattered light to form a defraction pattern and detecting the beat frequency which occurs in the form of changes in bright and dark fringes over the course of time. Indeed, this method permits extremely accurate determination of the velocity of a moving object such as a fluid. Nevertheless, it is incapable of determining the direction in which the object is moving.
Great advances have been made in recent years in the utilization of laser beams in the determination of physical phenomena. For example, there has been proposed a method involving utilization of the speckle pattern which can be produced by a laser beam. When a laser beam is directed onto the rough surface of a moving object, the light is scattered by the rough surface and different parts of the scattered light interfere with one another in the surrounding space to give rise to countless bright spots which form what is called a "speckle pattern". The speckle pattern method based on this principle determines the velocity of a moving object by making use of the proportional relation between the velocity of this speckle pattern and the velocity of the object itself.
The determination of the velocity of a moving object by means of the speckle pattern has so far been discussed by E. Jakeman of the Royal Radar Establishment (currently Royal Signals and Radar Establishment) of Britain in "The Effect of Wavefront Curvature on the Coherence Properties of Laser Light Scattered by Target Centers in Uniform Motion" [J. Phys. A: Math Gen., Vol. 8, No. 2 (1975)]. This publication, dealing with the method for determining the velocity of a moving object by causing the laser beam to be scattered by the object and correlating the speckle pattern produced in consequence of the light scattering, discloses an approach to the calculation of the velocity by means of the ordinary vector coordinate system. It says nothing about the quantity and direction of displacement of the moving object in the two-dimensional system. The optical arrangement which is involved in the determination uses one beam of incident light and two detection units. A dissertation discussing this subject more extensively is made by Ichiro Yamaguchi in a Japanese journal titled "Applied Physics", Vol. 46, No. 8 (1977), pp 803-812. This article starts with an introduction of the speckling phenomenon and characteristics and goes on to discuss application of the speckle pattern to astronomical observation, determination of the displacement, deformation, vibration, velocity and revolution of moving objects, determination of the surface roughness of objects and the like. In the publication, the author proposes to classify into several groups, by the type of optical arrangement, all the methods available for determining kinetic displacements of objects having a light diffusing surface by virtue of the deformation of speckle. To one of these groups belongs a "one light-flux and two-aperture image formation method" which comprises directing the laser beam onto a given light-diffusing surface, causing the scattered light to pass through two pinholes and impinge upon a diffraction plane so as to give rise to a speckle pattern, and then subjecting the formed speckle pattern to observation and analysis. There has also been proposed a "two light-flux and one-aperture image formation method" which comprises directing two laser beams, each at a specific angle of incidence, onto the light-diffusing surface, causing the scattered light to pass through one pinhole and impinge upon a diffraction plane so as to give rise to a speckle pattern, and subjecting the formed speckle pattern to observation and analysis. These two methods invariably aim to accomplish the determination of the displacement of a moving light diffusing object by measuring, in terms of the frequency of Doppler beat, the quantity of change in the phase difference of the two speckle patterns produced before and after the displacement of the light diffusing object. As clearly indicated in the respective pieces of literature cited above, these methods permit the determination of the displacement only in the direction perpendicular to the axis of the incident beam of light because the observation is made exclusively of the fringe intensity in the lateral displacement which is parallel to the direction connecting the two incident laser beams or the two pinhole apertures.
Of the two methods described above, the "one light-flux and two-aperture image formation method" has been discussed by H. Shimomura in the Glossary of Drafts, 30p-G-5 (1977), pp. 225 submitted to the 25th Meeting of the Applied Physics Society under the title of "Discussion on Optical System for the Two-Aperture Type Laser Doppler Speed Meter". This dissertation clearly indicates that the conventional two-aperture method is capable of only determining the velocity of a given moving object in one direction because the determination is based on the observation of Doppler beat.
An object of this invention is to provide a method for determining the velocity and direction of motion of a moving object, which method permits the determination to be carried out over a wide range of velocities with high resolvability and high reliability by causing the measurement of the velocity and direction of motion to be effected in two directional components of the moving object by making use of the moving speckle pattern to be obtained by the two light-flux and one-aperture image formation method, and an apparatus to be used for working the method described above.